(1) Field of the Invention
The present invention is directed to drag reducing devices for use in fluid mediums and in particular to fairings used to reduce drag and splashing between adjacent floating elements passing through water.
(2) Description of the Prior Art
Objects moving through a fluid medium, for example air or water, experience drag forces associated with their movement. Objects that move through fluid mediums include objects that are towed through the water from a vessel on or below the surface of the water. Examples of objects towed through the water include dredges, sonar devices and fishing nets. Drag forces work against the movement of the object, extracting speed and energy. Therefore, the objects are engineered to minimize these forces. The engineering efforts involve modifications to the materials, shape and surface characteristics of the objects. In addition to the objects, lines, ropes or cables are used for towing the objects, and the length of these can be considerable, resulting in a large amount of additional drag. Methods used to reduce the drag associated with these tow members include attaching fairings along the length of the member.
In general, fairings are formed like aerofoils or aerofoil sections that extend around a tow member such as a cable. Since the cable is flexible and the fairing sections can move or pivot about the cable, misalignments between the fairing sections and the direction of movement, i.e., direction of flow of the fluid medium around the cable, result in forces that push the cable to the sides, i.e., perpendicular to the direction of movement. These side forces result in a loss of control of the towing vessel. Also, the towing cable can be pulled out of handling equipment such as a sheave wheel. This can result in significant damage or complete loss of equipment. In addition to problems with misalignment, the fairing sections can migrate or move along the length of the cable, damaging the fairing sections.
Attempts at providing improved fairings for tow cables are illustrated in U.S. Pat. Nos. 4,836,122, 4,700,651, 4,398,487 and 4,075,967. These fairings, however, are for cables that pull objects below the water and are specifically adapted to reduce the drag on long, generally cylindrical cables. The cables are either long, unitary cables or cables constructed in sections. These sections, however, move through the water laterally not longitudinally. Thus, the hydrodynamic forces are forces perpendicular to the axis of the towed sections.
Applications exist where objects are arranged as a series of sections that are towed along the surface of the water or through the water. Each section is capable of articulation with respect to the adjoining sections, and the articulated sections move through the water in series, one behind the other. Therefore, disturbances in the water caused by a leading section affect trailing sections. In addition, the gap between adjacent sections that are used to provide the desired degree of movement between adjacent sections results in additional undesirable forces.
An example of an object arranged as a series of articulated sections is the multi-element buoyant cable antenna (MBCA), which is an antenna arranged to float horizontally on the sea surface and operate while being towed. Usual towing speed of such an antenna is about six knots. In one arrangement, the MBCA includes multiple pressure vessels which house the antenna, electrical, and connection components. The pressure vessels keep the contained components dry. Between the pressure vessels are a series of floats that are used to reduce the specific gravity of the overall assembly, allowing the antenna to protrude out of the water as much as possible. The floats are segmented to allow the assembly to conform to the water surface, e.g., waves, and to allow movement and turning of the assembly.
The spaces between the sections create drag and also cause splashing as the sections are towed through the water. Previous attempts at reducing drag have used hydrodynamic fairings placed between adjacent sections. These fairings were arranged as rectangular fabric pieces that were wrapped approximately one and one half times around the girth of each section. The rectangular fabric pieces, however, buckled when the sections articulated and become entrapped in the spaces between the sections, reducing the effectiveness of the fairing.
Therefore, the need exists for a fairing to be used between articulated sections to reduce the drag and splashing. In addition, the fairing should facilitate a sufficient range of articulation between adjacent sections while still providing the benefits of drag and splash reduction.